In semiconductor power devices, the structure and material of devices need to be selected to minimize ON resistance and maximize withstanding voltage. In the related art, the semiconductor power device is manufactured using silicon as a semiconductor material. A site where the electric field is concentrated in a terminal portion of the device has been designed to reduce the electric field using a ring structure of a p− type layer, or a PN junction, which is so-called junction termination extension (JTE)), formed on the surface so as to obtain a high withstanding voltage.
In the related art, a high withstanding voltage is obtained, for example, by minimizing the ON-voltage of the Schottky barrier diode and forming a p− type layer (so-called, resurf layer) as the JTE in a manner of continuously, outwardly extending from part of the Schottky electrode to deplete the p− type layer during a reverse bias and reduce the electric field at the end of the Schottky electrode. The withstanding voltage principally depends on a depthwise integral value of the concentration of the p− type layer, that is, on the dose amount of ions used to form the p− type layer. In order to obtain an ideal withstanding voltage, the dose amount is required to represent a value approximate to ∈Ec/q where Ec denotes a breakdown electric field intensity, ∈ denotes a dielectric constant, and q denotes an electric charge elementary quantum.
In recent year, in order to dramatically improve performance of power devices using silicon, power devices using silicon carbide (SiC) have been developed. The silicon carbide is a sort of wide-bandgap semiconductor, and has a breakdown electric field intensity 10 times larger than that of silicon. Therefore, it is possible to overcome a trade-off problem between the withstanding voltage and the ON-resistance of the semiconductor power devices. As in the devices using silicon, even in high-withstanding voltage semiconductor devices using silicon carbide, the JTE is formed on the surface to obtain a high withstanding voltage.
However, since silicon carbide has anisotropy in the breakdown electric field intensity, the electric field at the end of the JTE is deviated at a tilt with respect the C-axis direction where the breakdown electric field intensity is largest. Therefore, the withstanding voltage is significantly deteriorated. Typically, the breakdown electric field intensities Ec1 and Ec2 in the C-axis (<0001> orientation) and the A-axis (<11-20> orientation) perpendicular to the C-axis, respectively, can be expressed as the following equations.Ec1=2.70×106(Nd/1016)0.1 [V/cm]  (1)Ec2=2.19×106(Nd/1016)0.1[V/cm]  (2)
where, Nd denotes a doner concentration in an epitaxial film formed on the SiC substrate, and the symbol “-” denotes a bar attached over a number according to a crystallographical principle.
Due to anisotropy of the breakdown electric field intensity, the breakdown electric field intensity in the A-axis direction is lower than the breakdown electric field intensity in the C-axis direction by 10% or more. If this reduction is calculated in terms of the withstanding voltage, it corresponds to 30% or more.
As described above, in the silicon carbide semiconductors, it is desired to effectively prevent deterioration of the withstanding voltage caused by anisotropy of the breakdown electric field intensity without an increase in costs.